COVID-19 Breakthrough Infections among Patients Aged ≥65 Years in Serbia: Morbidity and Mortality Overview

BACKGROUND: Vaccines against severe acute respiratory syndrome coronavirus 2 have shown effectiveness in the prevention of COVID-19. Breakthrough infections occur, and age has been shown to be one of the dominant risk factors for poorer outcome. This research focuses on characteristics of breakthrough infections in older adults. METHODS: This retrospective study was conducted for four months (March-June 2021) in the autonomous province of Vojvodina in Serbia on 11,372 patients using reverse-transcription polymerase chain reaction or antigen-detection rapid diagnostic tests verifying COVID-19 in those aged >= 65 years. Demographics, comorbidities, disease severity, and final outcomes were evaluated in fully vaccinated compared to unvaccinated individuals. Individuals were divided into younger-old (65-74 years) and older-old (>= 75 years) age groups and differences between those groups were further evaluated. Binary logistic regression was performed to identify independent predictors of poor outcome. RESULTS: By the end of the research, 51.3% of the population of APV 65-74 years, as well as 46.2% of those older than 74 years, were vaccinated. From the acquired sample, 17.4% had breakthrough infection. Asymptomatic forms were higher in both age groups of vaccinated vs. unvaccinated (3.9%-younger-old, 6.3%-older-old vs. 2.9%-younger-old, 3.9%-older-old). The same results were registered with mild symptoms (82.1%-younger-old, 68.1%-older-old vs. 76.3%-younger-old, 57.5%-older-old) (p < 0.001). The case fatality ratio of the vaccinated population was smaller than the unvaccinated population in both groups (3.1% vs. 7.9%-younger-old; 11.4% vs. 22.5%-older-old) (p < 0.001). The odds ratio for poor outcome in unvaccinated individuals was 2.3 (95% confidence interval, p < 0.001) for the total sample. CONCLUSIONS: An increase in asymptomatic and mild forms, as well as decrease in severe or critical forms and poor outcomes, were noted in the vaccinated population. Choosing to avoid vaccination against SARS-CoV-2 may increase the chance of poor outcome in older individuals.